A weblog for professionals in electrical, electronic, mechanical and software engineering with content provided by the members of the Long Island Consultant's Network.

April 2009

April 27, 2009

There was this signal generator whose output frequencies were derived by a phase-locked-loop that was referenced to a 10 MHz oscillator. The specification control drawing (SCD) for that oscillator called out that the oscillator's harmonics of 10 MHz not be greater than 60 dB below the level of the 10 MHz signal itself, a requirement that was met quite nicely by oscillators purchased per that SCD from oscillator company "B".

Happiness reigned throughout the land.

Then one day, someone in the purchasing department decided that company "B" was too costly to buy from and went instead to company "A" who had quoted a lower price. With the new company "A" oscillator, the signal generator's output was suddenly filled with all kinds of nasty intermodulation products which would simply vanish when an older company "B" oscillator was put back in.

Unhappiness now reigned throughtout the land.

All of this was officially seen as an "engineering problem". Engineering was officially accused of being irresponsible for not caring about costs in advocating a return to company "B" as the oscillator's supplier.

April 22, 2009

There was a very old issue of Mad Magazine that described a device whose fuel tank was calibrated in liters per cubic meter in order to improve that device's confusion factor. Thus was born (sort of) a lifelong interest in metrology.

Now consider the measurement of velocity. My car's speedometer scales of miles per hour and kilometers per hour just don't do it somehow, so let me suggest something to liven up this kind of accounting.

We recall that one furlong is one-eighth of a mile (660 feet and also 220 yards, if that's of any interest).

We also recall that one fortnight is fourteen days which comes to 14 x 24 = 336 hours.

Therefore, one furlong per fortnight very nearly equals 372 micro-miles per hour.

Now, if my speedomter could be calibrated in furlongs per fortnight, that would be a real gas and it might even have a public safety aspect.

Fifty-five miles per hour would come to 147,800 furlongs per fortnight. One might think fifty-five miles per hour to be nerve wrackingly slow (Have you driven on Route 135 lately?), but a speed measured as 147800 anythings might be more satisfying.

April 18, 2009

The project was a broadband amplfier used in a military design signal generator. The amplifier consisted of several single transistor gain stages which worked very nicely as long as the cover of the aluminum housing was left off, but when the cover was put on, the amplifier would develop parasitic oscillations at approximately 1 GHz.

My assignment was to fix the amplifier without changing anything. Management had told the end customer that all was well with the signal generator's design and they didn't want to have to justify any new Engineering Change Notices.

The housing was a rectangular box, about fifteen inches long, three inches high and six inches wide with the transistors of each gain stage located pretty much in the center of the six inch dimension. All of the components were mounted on insulated terminals. There was no circuit board.

With an intended operating bandwidth of 40 MHz, the designer had chosen 2N918 transistors, a pretty good choice I thought given their ft rating of 600 MHz. How then did the 1 GHz activity come about?It turns out that the six inch dimension is just about a half-wavelength at 1 GHz. Those 2N918 transistors must have been somewhat speedier than was demanded of them by their specification because, when the cover was put on the chassis, the transistors were supporting 1 GHz oscillations inside of the closed box which was acting as a waveguide. It looked like each transistor was launching signals into the waveguide structure and picking signals up from the others.

The only remedy I could come up with was to violate management's orders and make a change. I put a ten ohm resistor in series with each 2N918's base lead to kill their VHF/microwave capability. The amplifier still worked with its cover off, but now it kept working with its cover on too.

April 06, 2009

The impedance presented by a length of transmission line is a constant resistance value "R" when the characteristic impedance value of that transmission line is "R" and a load resistance "R" is connected, but if the load value is not matched in that way, then the presented impedance varies as a function of the cable's length.

Let's call the transmission line a cable and let's assume a load "R" that does NOT equal the cable's characteristic impedance.

If we choose a particular frequency, look at the cable as a cascade of inductance and capacitance values per differential bits of unit length and do a repeated calculation, over and over and over again, of the presented impedance, we can plot that impedance graphically and find out at what cable length the presented impedance again equals the load resistance. That cable length is one-half wavelength at the frequency of the observation.

Double that length to find the full-wave length and then compare that to the free-space wavelength at the chosen frequency. The ratio of the two is the velocity factor of the cable.

April 05, 2009

To make it possible for contributors to upload images and files, I had to make a change that opens up the possibility of changing the posting status. Please to not change the default setting in that box. Leave it set to "Draft".

It was like running into a verbal bridge abutment. I couldn't understand anything they wrote!

I have no doubt that the authors knew about making French toast and that they'd written words which they thought would effectively convey the necessary steps to the reader, but they didn't do it in any way that this reader could comprehend.

April 02, 2009

There are so many changes taking place -- at such a fast pace -- related to the Internet that it is truly dizzifying. Every day I read about new versions of smart phones -- and now we have netbooks -- that take advantage of Internet voice and data communications. Just the other day I read about the upcoming introduction of streaming TV to a Blackberry (see http://tinyurl.com/dlqnfk ). I can't imagine myself watching TV on my smartphone. But then again, I was originally dubious about checking my e-mail and reading the NY Times on my smartphone -- and I do that every day now.

I wonder if being a consultant, or at least acting like one, is the way of the future. Let me explain….

For centuries, craftsmen would work at their trades and teach them to their children who, in turn, would pass on those skills to their children and so on. Today, there are probably very few skills where this still applies. In many professions, especially in technology, the transition period from state-of-the-art to old hat becomes shorter and shorter necessitating a never-ending learning process in order to keep current on the latest technologies.

Changing professions has also become commonplace. I don’t recall the exact numbers, but a number of years ago I read that most people make at least one career change in their lifetimes. At that time, lawyers had the highest turnover rate. Much of this is due to disillusionment with the chosen professions, but I suspect that declining job opportunities is also a significant factor.